Vibrating flow devices, such as, for example, densitometers or Coriolis flow meters, are available in various sizes and flow capacities. Densitometers typically have one or more conduits of straight, U-shaped, or an irregular configuration which are vibrated transversely by a drive at a resonance frequency for purposes of determining the density of a substance within the densitometer. The particular resonance frequency at which the one or more conduits vibrates is partially determined by the density of the substance within the one or more vibrating conduits. Accordingly, as the density of the substance within the one or more vibrating conduits changes, the frequency at which resonance occurs will change. Therefore, using well known time-tested principals, the particular frequency at which resonance occurs may be used to compute the density of the substance within the one or more conduits.
Densitometers include one or more electronics that transmit a sinusoidal drive signal to a drive, which is typically a magnet/coil combination with the magnet typically being affixed to the flow tube and the coil being affixed to a supporting structure or to another flow tube. The drive signal causes the drive to vibrate the one or more conduits at the resonance frequency. For example, the drive signal may be a periodic electrical current transmitted to the coil. A pick-off detects the frequency of vibration of the one or more conduits and generates a sinusoidal pick-off signal representative of the motion of the flow tube, including the frequency of vibration of the flow tube. The sinusoidal pick-off signal is transmitted to the one or more electronics and used by the one or more electronics to determine the frequency at which the one or more conduits vibrate. If the one or more conduits are vibrating at the resonance frequency, the electronics may use the pick-off signal to determine the density of the substance within the tube. If the one or more conduits are vibrating at a non-resonance frequency, the electronics may adjust the drive signal transmitted to the drive so that the one or more conduits vibrate at the resonance frequency.
Accordingly, using well known principals, vibrating densitometers have been used for years to measure the density of substances. Vibrating densitometers constructed with a single drive and a single pick-off, however, have in the past been incapable of detecting one or more flowing characteristics of the substance within the conduit, such as, for example, whether the substance within the one or more conduits is flowing, the direction in which the substance is flowing, or the mass flow rate of the substance. In particular, in certain applications it may be desirable to determine whether the substance is flowing. In order to detect the presence of flow, changes in the time shift between the frequency of vibration induced by the driver and the frequency of vibration detected by the pick-off may be used. Those skilled in the art appreciate that the time shift equals the phase difference between the frequency of vibration induced by the driver and the frequency of vibration detected by the pick-off divided by the frequency of vibration induced by the driver and the frequency of vibration detected by the pick-off.
Heretofore, in single drive and single pick-off densitometer systems, however, the frequency detected by the pick-off is phase-locked to the frequency applied by the drive. Therefore, as flow occurs or changes, the time shift between the frequency of vibration applied by the drive and frequency of vibration detected by the pick-off does not change as flow occurs or changes. Accordingly, in the past, at least two pick-offs have been required for the detection of the presence of flow, detection of the direction of flow, and the determination of the mass flow rate of the substance.
The present invention is directed to overcoming this and other disadvantages inherent in prior art densitometers.